Transportation vehicles, such as automobiles, watercraft and aircraft, are increasingly prepared using low weight materials to reduce fuel consumption. To achieve this purpose honeycomb structures are employed to replace solid structures more and more frequently. The size of the void cells in a honeycomb structure influences the mechanical properties of the structure. The bigger the size the greater the weight reduction but the greater may be the loss of mechanical strength. Void cells in honeycomb structures may typically range from 5 mm up to 10 cm in at least one or all three dimensions. There is the desire to employ honeycomb structures with larger void cells and to find solutions to counteract the potential loss of mechanical strength. One approach is to partially or completely fill some or all of the void cells of the honeycomb structure with filler materials. Epoxy resin based compositions may be used for this purpose.
In U.S. Pat. No. 6,635,202 to Bugg and Hayes, a thermally foaming powder is disclosed that can be used as filler in honeycomb panels. The thermally foaming powder contains Araldite GY260 (a bisphenol A epoxy resin), dicyandiamide, chlorotoluron, 3,3′-dimethyl 4,4, diamine dicyclohexyl methane, Expancel 551 DU (unexpanded micro-spheres), and Cabosil TS720. The powdery precursor is obtained by mixing the starting compounds at temperatures below the onset temperature of the epoxy compound and below the temperature where expansion of the unexpanded thermoplastic microspheres occurs.
The powders are filled in a honeycomb structure and heated thereby expanding, foaming and curing the powder. The powders of U.S. Pat. No. '202 may not be easily used in vertical bonding geometries, which limits their applicability.
Moreover, when using curable epoxy resins as core fillers the curing reaction is typically exothermic. Generally, this is not a problem in standard applications of curable epoxy resin formulations where the epoxy resins are used to bond components together. In such applications curable epoxy resin compositions are typically applied as thin layers between the substrates to be joined and bonded. However, the exothermic reaction may become noticeable when curing thick layers of epoxy resins as may be the case when using curable epoxy resins as filler materials, for example, in honeycomb structures. Due to the comparatively large size of the cells, the epoxy resin compositions may have to be applied in a thickness of greater than 30 or even greater than 50 mm. Heat generated by the exothermic curing reaction of substantial amounts of epoxy resins may thermally damage the epoxy resin composition and impact on its mechanical properties. It may also affect the honeycomb structure or components attached to the honeycomb structure. Therefore, there is a need for providing curable epoxy resin compositions that develop little exothermic heat during the curing reaction.
In international patent application WO 2006/071820 an epoxy-based filler composition is disclosed. WO 2006/071820 teaches to avoid amine based hardeners for overcoming exothermic heat generation upon curing the epoxy composition.
In addition to overcoming the problem of heat generation, curable filler compositions are also desired that can be cured to provide compositions having good compressive strength not only at room temperature but also at elevated temperatures, such as for example 135° C. or even at 150° C.
Furthermore, the curable filler compositions desirably are extrudable pastes to allow easy application to honeycomb structures.
Finally, curable filler compositions are desired that have in addition to the properties mentioned above a good shelf-life at room temperature.